Update cache directory and PATH to use the correct folder

[JasonWhall]

Fixes #64

Sets the correct folder to prepend to path and cache in the tool path as the Pulumi binary exists in a pulumi sub-folder in the extracted tar.gz, similar to Windows

Thank you for contributing! Before submitting your PR, can you please confirm that you have done the following?

• [x] If you changed the values of the properties name, publisher, and galleryFlags in the vss-extension.json file, you have reverted them.
• [x] If you changed the values of the properties id, name, author, you have reverted them.
• [x] I have reverted any changes to the version number components in both vss-extension.json and task.json, OR I didn't change them.

[praneetloke]

/azp run

[azure-pipelines[bot]]

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